In 1965, after walking in the Selma-to-Montgomery civil-rights march with the Rev. Dr. Martin Luther King Jr., Rabbi Abraham Joshua Heschel was at the Montgomery, Ala., airport, trying to find something to eat. A surly woman behind the snack-bar counter glared at Heschel — his yarmulke and white beard making him look like an ancient Hebrew prophet — and mockingly proclaimed: “Well, I’ll be damned. My mother always told me there was a Santa Claus, and I didn’t believe her, until now.” She told Heschel that there was no food to be had.

In response, according to a new biography, “Spiritual Radical: Abraham Joshua Heschel in America, 1940-1972” by Edward K. Kaplan (Yale), Heschel simply smiled. He gently asked, “Is it possible that in the kitchen there might be some water?” Yes, she acknowledged. “Is it possible that in the refrigerator you might find a couple of eggs?” Perhaps, she admitted. Well, then, Heschel said, if you boiled the eggs in the water, “that would be just fine.”

Of course Heschel, with his rabbinic features, could not have looked too much like the jolly gentleman expected to visit homes late Christmas Eve. But the spirit evident in this anecdote must have served him well over the years as he taught aspiring rabbis, met with Pope Paul VI and became a leader in the civil-rights, anti-Vietnam War and interfaith movements. At his death in 1972 he was one of this country’s best-known Jewish figures.

This year’s centennial of Heschel’s birth, commemorated by the new biography and a conference this month at the Center for Jewish History in Manhattan, takes place in a very different world. Surely no one today could write, as he did in his landmark 1955 book, “God in Search of Man,” that there is an “eclipse of religion in modern society.” If anything, there is no escape from talk about faith. Nor is the relationship between religious convictions and political activism as simple as it might have once seemed.

But in turning again to Heschel’s writings, which had such an impact in the 1950s and ’60s, I was startled by how much vitality they still possess. The Heschel biography shows how many people were touched by his charismatic persona; the potential for such contact is evident in his own books as well.

Admittedly there are times when Heschel can seem sentimental or, as in his early book “The Earth Is the Lord’s,” can romanticize the past. He turns the lost world of his fathers — the communities of Eastern European Hasidim and their rabbis — into an almost utopian realm. The scholarly skepticism of his colleagues at the Jewish Theological Seminary in New York, where close textual analysis was more eagerly embraced than Heschel’s inspirational philosophy, does not always seem unmerited.

But no modern Jewish thinker has had as profound an effect on other faiths as Heschel has; the Protestant theologian Reinhold Niebuhr said he was “an authoritative voice not only in the Jewish community but in the religious life of America.” Nor has any Jewish theologian since Heschel succeeded in speaking to such a wide range of readers while rigorously attending to the nuances of Judaism.

Some of this uniqueness can be felt in the way Heschel approached the woman in the airport. Her mockery is defused, the interaction shifted to the mundane. It is as if Heschel were saying: “I understand I’m not what you’re used to. But I’m prepared to meet you casually, accepting your comparison to a make-believe figure. But surely you can see that your anger is not justified?”

The confrontation dissolves into a conversation, the hostility into humor. The temptation would have been to do the opposite — to chide or stiffen with resentment — particularly given Heschel’s own personal trials. A yeshiva student in Poland, he rebelled not by becoming a secular Jew but by getting a doctorate in theology and philosophy from the University of Berlin. He fled the Nazis (who murdered one of his sisters and caused the death of his mother) but never found a comfortable intellectual home in the United States — neither during his early years at the Hebrew Union College in Cincinnati nor during his long career at the Jewish Theological Seminary.

Perhaps that history of trauma and dislocation made him more alert to disruptions in others. But in the airport conversation, Heschel gently found a way to dispose of opposing social roles — the protesting rabbi scorning racism, the put-upon woman threatened by difference — and establish the beginnings of an understanding.

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The quest for common ground seemed to inspire his theological explorations as well. Heschel, influenced by German phenomenology, was preoccupied with experience rather than fact, with poetic evocation rather than explication. At the seminary he was a professor of Jewish ethics and mysticism. He was intent on communicating the incommunicable, exploring the ineffable.

In a book about the Sabbath he describes Judaism’s focus on the sanctification of time. In referring to God he does not imagine an Aristotelian prime mover but a transcendent being who needed humanity to fulfill himself. In thinking about humanity Heschel asked, “What way of living is compatible with the grandeur and mystery of life?” Such speculations crossed doctrinal boundaries and helped make him an important ecumenical force. “No religion is an island,” he wrote.

But amid this fervor Heschel was also a follower of Jewish laws, putting an emphasis on ritual and actions, not just on devotion and belief. This was also the source of Heschel’s ethical perspective: Every deed poses a problem with moral and religious implications.

“Judaism,” he wrote, “is not a science of nature but a science of what man ought to do with nature.” No act is permitted to escape scrutiny.

These poles of devotion and deed combined in Heschel’s activist politics in the 1960s, resulting in positions that still tend to determine his reputation. At the recent conference one speaker wondered where a contemporary Heschel might be found, someone prepared to take a stand against the wars in Afghanistan and Iraq the way Heschel did against the Vietnam War.

But those political positions are Heschel’s least compelling. In the civil-rights movement his moral stance was clear, but in discussing the Vietnam War, in which the issues were more complex, his statements, affected by the temper of the time, became less revealing, replacing evocation with hortatory proclamations modeled on the biblical prophets. “There is nothing so vile as the arrogance of the military mind,” he wrote. He used the word “evil” to allude to the “insane asylum” around him.

The result was a kind of theological politics. The prophets claim such declarations to be divine revelations, but in the earthly realm acts and consequences must be assessed, their complications untangled. No doubt political issues are sometimes so urgent they demand theological treatment. But there are risks in such a confusion of realms.

Theological politics tends to eliminate distinctions and is impatient with differences, empathy and argument. Had those kinds of views shaped Heschel’s perspective at the airport, instead of accepting the eggs from the offending woman, he might have thrown them in her face.

Connections is a critic’s perspective on arts and ideas.

A version of this article appears in print on , on Page E1 of the New York edition with the headline: A Rabbi of His Time, With a Charisma That Transcends It. Order Reprints|Today's Paper|Subscribe